In recent years, the research and development of display devices using self-emitting elements represented by electroluminescence (EL) elements and the like instead of liquid crystal displays (LCD), which include pixels using liquid crystal elements, has advanced. These light-emitting devices utilize advantages such as high-resolution due to the fact that they are self-emitting, they have a wide viewing angle, and they are thin and lightweight because they do not require a backlight, and therefore they are expected to have a wide use as display screens for mobile telephones and as display devices.
Also, increasing sophistication is demanded in display devices themselves due to the diversification of the purposes of use of such as mobile telephones, and color display devices that conduct multicolor display are already being widely used.
FIG. 5(A) shows an example of a common color display device. A pixel portion 501, a source signal line drive circuit 502 and a gate signal line drive circuit 503 are formed on a substrate 500. The input of signals to the drive circuits and the supply of an electrical current to the pixel portion 501 are conducted from the outside via a flexible printed circuit (FPC) 504.
In FIG. 5(A), the portion represented by the dotted line frame 510 is one pixel. FIG. 5(B) shows an enlarged view of part of the pixel portion 501. Each pixel respectively includes a source signal line 511 for inputting an image signal, a gate signal line 512 for conducting line selection, a current supply line 513 for supplying an electrical current to an EL element 516, a transistor 514 for switching, a transistor 515 for driving, a power line 517 and a retention volume 518. There is description in Patent Document 1 in relation to a circuit configuration where one pixel is configured using two transistors and which drives a load (here, the EL element is used as an example).
As one method that conducts multi-gradation display in such a display device using EL elements, there is a driving method where digital gradation and time gradation are combined (see Patent Document 2). According to this method, there is the advantage that it is difficult for fluctuations in the characteristics of the elements to influence image quality because it suffices as long as two states, the light-emitting state and the non-light-emitting state, of the EL elements can be controlled.
(Patent Document 1) Japanese Patent Laid-open No. 2000-147569
(Patent Document 2) Japanese Patent Laid-open No. 2001-343933
In the case of conducting color display, the respective emissions of R, G and B are controlled using, for example, three adjacent pixels represented by the dotted frame 520 in FIG. 5(B), and multicolor display is conducted by mixing these colors. In other words, three pixels are required for a 1-bit display.
In comparison to pixels in the case of conducting a monochrome display, the pixels of a color display device with which multicolor display is possible have many constituent elements, and the area occupying the display region is also large. Thus, the aperture ratio drops. In order to obtain a desired luminance, it is necessary to raise the emission luminance by the amount that the aperture ratio has dropped. In order to raise the emission luminance, it is necessary to raise the current density per pixel, but this leads to a reduction in the life of the EL elements.